The unpressurized reservoir is at atmospheric pressure or at a low pressure, no greater than the boost pressure.
By means of its hydraulic motor, such a circuit serves, for example, to drive a mass, such as the turret of a vehicle, e.g. a hydraulic digger, or such as a tired vehicle whose wheels are driven by hydraulic motors.
It is an open circuit, and in operation, the feed main duct is connected to the delivery orifice of the pump, while the discharge main duct is connected to the unpressurized reservoir. In a manner known per se, the selector can make it possible to invert the connections so as to cause the motor to operate in both operating directions.
For example, the hydraulic motor is of the type having radial pistons.
In order to brake or stop the motor, the selector is placed in the isolation position so as to isolate the main ducts from the pump and from the reservoir. In other words, the feed and the discharge of the motor are both interrupted.
In general, it is desired to limit leaks in hydraulic circuits, and so modern hydraulic components, in particular those of the motor, are increasingly fluid-tight. This makes it possible, in particular, to brake with precision and to prevent the mass driven by the motor tending to start moving again after it has been stopped, e.g. under the effect of its inertia, in particular when said mass is on a slope.
In addition, the boost duct is connected to a discharge main duct in order to avoid cavitation phenomena in said duct. This connection is achieved by disposing a check valve on the boost duct, which check valve allows fluid to flow only in the direction going from the boost pump towards the discharge main duct.
That arrangement thus makes it possible to boost the discharge main duct with boost fluid.
The main duct which, when the circuit is in an operating state, is at low pressure, can also receive fluid from the duct at the high pressure, when said high pressure is deemed to be excessive. That transfer of fluid is made possible by means of the presence of the pressure limiters for protecting the circuit against excessive pressure, which pressure limiters are disposed between the main ducts. That transfer of fluid achieves additional boosting which does not go through the check valves used for the boosting from the boost duct, so that said check valves can have small dimensions.
In general, the pressure limiters are disposed in the vicinities of the main orifices of the motor. When the main ducts are isolated from the pump and from the reservoir for braking the motor, the motor and the two above-mentioned pressure limiters form a closed loop containing a relatively small volume of fluid, which volume is only slightly larger than the active cubic capacity of the motor.
It has been observed that, when the motor is stopped by isolating the main ducts from the pump and from the reservoir, an increase in the volume of fluid present in said closed loop can occur. That increase in volume is due at least in part to an increase in the temperature of the fluid present in said loop, the energy necessary for the braking dissipating therein in the form of heat. In addition, it can happen that, in its isolation position, the selector does not isolate the main ducts and the pump in completely fluid-tight manner, so that a small quantity of fluid coming from the pump continues to feed the above-mentioned closed loop.
Said increase in volume, which can occur during braking, gives rise to an increase in the fluid pressure in the above-mentioned closed loop, which generates load on the components of the motor, in particular on the bearings via which the rotor and the stator rotate relative to each other. Ultimately, that phenomenon can lead to premature wear on certain components of the motor, in particular its bearings.